This invention relates to the manufacture of polymeric buns from a liquid polymer which expands upon reaction and, more particularly, to a novel and highly effective method and apparatus for manufacturing flat-topped polyurethane buns, and to buns made by the method or apparatus.
It is conventional practice to manufacture cured polyurethane and similar plastics by dispensing a liquid polymer which expands upon reaction onto a support surface and moving the polymer along a conveyor while the polymer expands to form a long, cured bun. The bun is subsequently sliced or otherwise processed as necessary, depending on the intended use of the plastic.
The buns formed by the conventional techniques tend to have rounded tops, since the plastic expands outwardly in all directions. If the sides of the bun are unsupported or poorly supported, the bun is typically flat on the bottom, and the sides and top merge to form a bun that is generally semicircular in cross section. If the sides of the bun are well supported (as for example with a paper or plastic substrate), the bun is flat on the bottom and sides and has a rounded top.
A patent to Taft, Jr. et al U.S. Pat. No. 3,013,924 discloses dispensing polymeric material such as polyurethane onto a supporting liquid that has a specific gravity greater than that of the polymer so that polymer floats on the liquid. However, the polymer is formed into a thin web rather than a bun. The patent also discloses two parallel movable edge-defining walls for limiting the lateral flow of the polymer across the liquid. However, the walls move in a direction parallel to the surface of the liquid and not in a direction which facilitates expansion of the polymer into a flat-topped bun.
All of the conventional techniques for manufacturing flat top polymeric buns from a polymer which expands upon reaction leave a great deal to be desired. The known techniques for manufacturing a bun which has a bottom, left and right sides and a top all of which are well defined and flat generally require complex machinery of a type not generally found in a typical urethane foaming plant. Moreover, conventional techniques and systems tend to be relatively expensive in practice, in terms both of the equipment required to practice the techniques and of the downtime required to service the equipment. The excessive downtime results largely from the tendency of the polymer to adhere to the parts of the equipment with which it comes in contact, thereby causing a gradual buildup of plastic on the equipment which eventually seriously degrades both the equipment and the manufactured bun. A further drawback of prior art devices for manufacturing flat top buns lays in their inability to foam different types of formulations. Thus, devices that are capable of making flat top buns from polyether foam formulations cannot operate with polyester formulations. These disadvantages are overcome by the present invention as set forth herein.